This invention relates to a spark plug for use in an internal combustion engine for an automobile or the like.
Recently, in the automobile industry, positive efforts have been made in order to achieve a high performance design of an engine (particularly, a low fuel consumption and a high output power). There is a tendency that the air/fuel mixture to be supplied to the engine is leaner, and there is also a tendency that the compression ratio is higher. Therefore, the voltage (hereinafter referred to as "required voltage") for producing a discharge across a spark gap becomes higher with the same spark gap. Naturally, with the increase of the running distance, the spark gap increases because of the consumption of an electrode, so that the required voltage increases. The rate of increase of the required voltage is high in the type of engine having a lean air/fuel ratio and a high compression ratio. The voltage to be supplied to the spark gap is determined by coil characteristics, and therefore if the required voltage exceeds the coil generating voltage, a firing occurs. Two methods of overcoming this difficulty can be considered. Namely, one is to increase the generating voltage, and the other is to decrease the required voltage for the spark gap. The former method has a disadvantage that the voltage resistance at the connecting portions of a distributor, a high-voltage cord, the coil and the spark plug becomes less reliable.